Magnetic transducer with separable pole faces



Jan. 18, 1949. v w, WEST 2,459,299

MAGNETIC TRANSDUCER WITH SEPARABLE POLE FACES Filed July 20, 1946 2Sheets-Sheet 1 /2/ iii 5 2f /8 22 INVENTOR. W/LL/AM f. 14 1557" Jan. 18,1949. w, w s-r I 2,459,299

MAGNETIC TRANSDUCER WITH SEPARABLE POLE FACES Filed July 20, 1946 2Sheets-Sheet 2 INVENTOR. W/LL/AM P. W527 Patented Jan. 18, 1949 MAGNETICTRANSDUCER WITH SEPARABLE POLE, FACES William P. West, Philadelphia,Pa., assignor to Philco Corporation, Philadelphia, Pa., a corporation ofPennsylvania Application July 20, 1946, Serial 'No. 685,049

llGlaims. l

The present invention relates to magnetic recording andv reproduction ofsound. Particularly, the inventionis concerned with sound recording andreproducing. systems in which a record medium such as a magnetic tape orwire is passed in contactwitha magnetic sound recording, reproducing orobliterating head. More specifically, the invention has to do withimprovements, in-the construction of such magnetic heads.

It is a Well recognized fact that when the magnetic. head isused'forrecording sound in systerns of the type above mentioned, acertain amount of demagnetization ofthe-record tape or wire takes placeas it.-leaves the head. This demagnetization may result in serious sounddistortions, especially noticeable at the higher-frequency, signals,Likewisait is a well recognized fact that when the ma netic; head isused for reproducing sound recorded-on the tape or wire, a certainamount of secondary or stray flux occurs which may. deleteriouslyeifectthe reproduction by producing spurious variations in the frequencyresponse curve. However,- as is generally lznown in the art, theresults. of demagnetization and the effects of' secondary or stray fluxare-negligible when the gap over which the magneticmedium tI2W81S;.iS:sufficientlyshort in comparison with; the shortest recorded wavelength.

With magnetic heads of the kind above referred to, it has been founddesirabletoconstruct the pole pieces of thin laminations'in order toavoid the effects. of eddy currents. Also, with suchmagnetic heads, ithas been foundthat best results are obtainedif the gapwhichis providedbetween the adjacent ends of, thepQle: pieces at the effective portionof the head, is located exactly at the point where the tape or wire istangent to thcagap. Moreover, the length of the gap, that is, thedistance between; the adjacent endsofthepole pieces atthe effectiveportion of the head, must be small and carefully controlled in order toobtain the-bestresponse at the higheraudio frequencies. Theserequirements, together with the fact that the component parts of themagnetic head are of small dimensions, create serious manufacturingproblems which have heretofore complicated assembly and made productiondifficult.

It is therefore the-primary object of this invention to improvetheconstr-uction of magnetic heads of the general type mentioned sothatthey can be readily manufactured and easily assembledin massproduction.

Another object of the invention is to provide a magnetic head soconstructed that it can be made in two major parts. Such a constructionhas the advantage that the. part which requires the greatest accuracycan be made as a unit for ready association with the other part tocomplete the device. This construction further has the advantage thatthe part which is most susceptible to wear can bereadily removed as aunit for repair or replacement.

Still another and more specific object of the invention resides in theprovision of an improved pole-piece unit for. amagnetichead. Theprovision of such a unit facilitates. assembly and makes possible therapid mass production of inexpensive but good magnetic heads.

These and other objects, and the manner in which they are attained, willappeanfrom the following description based on the accompanying drawingswhich illustrate a preferred embodiment of the invention.

In these drawings:

Figure 1 is a front elevational view of a magnetic head constructed inaccordance with the invention, the device as shown being approximatelythree times the actual size;

Figure 2 is a vertical sectional view taken substantially on line 2-2 ofFigure 1;

Figure 3 is a perspective view on a greatly enlarged scale illustratingthe construction of the pole-piece unit;

Figure 4 is an end elevation of the companion pole pieces employed inthe unit shown in Figure 3;

Figures 5, 6, '7 and 8 are explanatory views illustrating a series ofsuccessive steps which are preferably followed to produce the laminatedcompanion pole pieces;

Figure 9 is a perspective of the support for the pole pieces;

Figure 10 is a fragmentary view illustrating a portion of the structureshown in Figure 1, but showing an element added to the pole piecesupport for the purpose of insuring proper mounting of the pole pieceunit.

Referring more particularly to the drawings, the magnetic headdesignated, generally, by the reference character I!) in Figures 1 and2, includes a yoke structure H and a pole-piece unit [2. As shown, theyoke structure has the form of a core consisting of material of highpermeability. Preferably, this core is of laminated construction toreduce eddy current losses, as is customary in cores used fortransformers, chokes and the like. The core is provided with coils l3and i4 and with a gap I5 (Figure 1). Preferably, the coils I3 and M aremounted on oppositely disposed branches l6 and I! of the core, and thegap i5 is located in another branch l8 of said core. The gap i5 is of asize and shape to receive and to retain the pole-piece unit E2 in fixedposition with respect to the yoke structure. Further, the gap i5 is soarranged that the effective portion of said unit, as indicated at E9 inFigure 1, protrudes slightly beyond the marginal edges of the yoke inorder to provide good magnetic coupling between said eifective portionand the travelling record medium shown diagrammatically at M and whichmay comprise a magnetic tape or wire. Also, the gap is such that an airspace, as represented at in Figure 1, is maintained between the adjacentends 21 and 22 of the yoke Where these ends do not contact thepole-piece unit.

As is more clearly shown in Figure 3, the polepiece unit i2 essentiallycomprises substantially C-shaped pole pieces 23 and 24 and a generallycylindrical support 25 therefor. The pole pieces consist of a pluralityof laminations in order to avoid the effects of eddy currents, and eachlamination is made of suitable material having the magnetic propertiesnecessary to obtain the desired result. It has been found that materialssuch as high permeability silicon-iron alloys are most suitable if thehead is to be used for sound recording or obliterating purposes, andthat materials such as high permeability nickel-iron alloys may be usedto advantage if the head is to be employed to reproduce recorded sounds.

In practice, the laminations are coated with varnish or a like substancewhich not only contributes toward reduction of eddy current losses butalso efliciently acts to bind the laminations together so that unitaryand rigid C-shaped pole pieces as seen in Figure 4, are obtained.

A quick and efficient way of mass producing the companion pole pieces isillustrated in Figures 5 through 8. As shown in Figure 5, the end por--tion 26 of a long strip 27 of magnetic material suitable for theformation of the pole pieces, is

literating sound. As the winding progresses, varnish or a like substancemay advantageously be applied to the strip in the manner illustrated at30 in Figure 5. The strip may be fed from a reel (not shown) andtensioning means (not shown) of any suitable known construction may beemployed to maintain the strip under regulated tension in order toassure a compact spiral as shown in Figure 6. After the desired numberof turns have been made, the strip is cut so that end portion 3! of thespiral terminates in the vicinity of groove 29 in the mandrel 28. Withthe spiral still retained on the mandrel, a sleeve 32 is slipped 7 overthe spiral for the purpose of facilitating sub- The to eliminate thebent end portion 26 from the main convolutions of the spiral. The sleeveand mandrel sections are then removed, thus leaving the companion polepieces shown in Figure 4, which pole pieces may then be readilyassociated with the support 25.

As more clearly illustrated in Figure 9, the support 25 consists of asolid body of non-magnetic material having rounded surfaces and 35 aboutwhich the C-shaped pole pieces 23 and 2d (Figure 4) may snugly fit(Figures 1 and 3), and further having gap-defining sections 8? and 38against which the opposite extremities of said pole pieces may snuglyabut (Figures 1 and 3). It is to be noted that the gap defined by onesection 3'! is much longer than the gap defined by the remaining section38. Because of this construction it will be understood thatconcentration of a strong magnetic flux is insured at the narrower gapwhich constitutes the effective portion 28 hereinbeiore referred to.

In practice, the gap-defining section 3? is obtained by providing alongitudinal boss integral with and projecting radially of the supportbody for a distance substantially equal to the thickness of thelaminated pole pieces; and the gapdefining section 38 is provided bymeans of a shim (a strip of non-magnetic stock) embedded in the supportto project radially therefrom also for a distance substantially equal tothe thickness of the laminated pole pieces. Moreover, it is pointed outthat the thickness of the boss substantially corresponds to the width ofthe wide out 33, and the thickness of the shim substantially correspondsto the Width. of the narrow cut 34, which cuts are made during theformation of the pole pieces as above described. The use of a boss andof a shim in the manner stated makes it possible to determine accuratelythe length of the gap at the effective portion of the pole-piece unit.

Because the length of this gap controls the maximum frequency responsefor a given speed of the record tape or wire, accurate control of thegap length becomes an important factor. Moreover, the gap length isusually very small, in fact this length is generally of the order of onemil. Such a small gap is necessary to obtain maximum high frequencyresponse. These requirements have heretofore made the attainment of theproper gap length tedious and difficult. Because, in accordance with thepresent invention, the C-shaped pole pieces 23 and 2d and support 25therefor are so related that the pole pieces snugly embrace thesupport-surfaces 35 and 36, and the adjacent extremities of such polepieces snugly abut the gap-defining sections 3? and 38 on said support,control of the gap length at the effective portion of the pole-pieceunit becomes an easy matter. It merely involves use of a shim having athickness equal to the desired gap length and the formation of a bosshaving a width sufiicient to insure snug engagement between the faces ofthe shim and the extremities of the pole pieces adjoining said faces.

A suitable adhesive such as varnish or the like may be convenientlyapplied to the corresponding surfaces of the pole pieces and supportwhen these elements are assembled. This insures retention of the polepieces on said support and provides a rigid and compact pole-piece unitwhich can be readily inserted in the gap 15 of the core or yokestructure ii, to complete the magnetic head. Either before or after thepolepiece unit is mounted in the yoke structure, the

exposed surfaces of the pole pieces-and of the shim are-convenientlysmoothed out to insure a clean even area, forgood: contact: with themagnetic tape or wire.

In manufacturing the pole pieces, an extended strip of magnetic materialmay advantageously be wound on a long-mandrel, and an equally longsleevemay be slipped over the wound-strip so that by cuttingtheassembled sleeve, spiral and mandrel: transversely along the length,thereof, several pair of companion pole'pieceszof 3 desired length may:be obtained from; onewinding operation. Likewise; ,thezsupport may bemadein long lengths which may-,thenbercut into sections of required sizeto accommodatethenpole pieces;

As shown in Figure 10,-theboss31 on support 25 may be provided with. anyextension 310 adapted to be introducedin the gap between the adjacentend portions2|and22 of branch 18 of theyoke. This extension serves toguide the mounting of the pole-piece unit, l21in the gap I510f' theyoke-and to' prevent the mounted pole-piece unit from rotating withinsaid gap l5:beyond desired limits;

In view of the foregoing it will be appreciated that thecomponent partsof a magnetic head constructed in accordance with the present inventioncan be readily manufactured and assembled intocompact unitslandthat suchunits, in turn, can. be easily assembled without requiring particularskill. Moreover, because the invention makes it possible to manufacturethe pole-piece unit separately, the accurate and precise relationshipwhich mustexist between the twopole pieces-can besreadilyv attained.Particularly, the invention smfacilitatesthe control oftheair gap.length at theeffective. portion of. themagnetic head that the-provisionof a good magnetidheadbecomes a matter of mere, mechanical procedurerather than skillful achievementof-scientific calculations.

Although a, particular embodiment of the invention has been describedwith great particularity, it is to be understood that this has been doneby way of example only. Various changes in the details of constructionand in the combination and arrangement of parts may be resorted towithout departing from the spirit of the invention which is subject onlyto such limitations as are imposed by the prior art or are specificallycalled for in the appended claims.

I claim:

1. In a magnetic head, companion pole pieces constructed to provide twopairs of confronting end portions, a support for said pole pieces, ashim fixedly mounted on the support and disposed in engagement with oneof said pairs of end portions. and a boss arranged on the support anddisposed in engagement with the other of said pairs of end portions tomaintain the first-mentioned pair of end portions in engagement with theshim.

2. In a magnetic head, a pair of substantially C-shaped pole pieces anda generally cylindrical support therefor, said pole pieces having endportions disposed in confronting relationship, and said support havingmeans extending radially therefrom and arranged in engagement with saidend portions of the pole pieces to provide a gap of definite lengththerebetween.

3. In a magnetic head, a pair of substantially c-shaped pole pieces anda support therefor, said support consisting of a body having surfacescorresponding to the inner surfaces of the pole pieces so as to supportsaid pole pieces in such a manner that the end portions of one polepiece confront the'end: portions ;of the other pole piece, andgap-defining sections extending radially of said body anddisposed inabutting. engagement with said. end portions. of the said pole pieces.

4. In a magnetichead, a pair of substantially G-sha-ped pole pieces, a.generallyv cylindrical membersuppcrting said pole pieces so that theend-portions 1 of one'pole piece confront the end portions of; the otherpiece, a shim embedded inland extending-radially from the support andhaving, a portion in, abutting engagement with adjacent end portions ofthe pole pieces to providea gap therebetween, and a boss integral withand alsoprojecting radially from the support and having a portion inengagement with other adjacent end portions of the pole pieces toseparate saidother end portions and to maintain the firstmentioned endportions in engagement with the shim.

5. A magnetic head comprising a yoke structure including a agnetic corehaving a gap in oneportion thereof, a pole-piece unit received in saidgap and including companion pole pieces andv a support therefor, saidpole pieces having end portions arranged in adjacency, and said supporthaving means disposed in engagement with adjacent end portions of thepole pieces to provide a gap of definite length therebetween.

6. A magnetic head comprising a yoke structure including a magnetic corehaving a generally circular-shaped gap in one portion thereof, agenerally cylindrical pole-piece unit received in said. gapand includinga pair of substantially C-shaped polepieces anda support therefor, saidpole pieces having end portions arranged in adj acency, andsaid supportconsistingof a body provided-with rounded surfaces in concentricengagement with the inner curvature of the pole pieces and alsoprovidedwith gap defining sections extended radiallyof-said body and in abuttingengagementuvithv said endportions of, the pair of pole pieces.

7. A magnetic head comprising a yoke structure including a magnetic corehaving a generally circular-shaped gap in one portion thereof, agenerally cylindrical pole-piece unit received in said gap and includinga pair of substantially C-shaped pole pieces and a generally cylindricalmember supporting said pole pieces so that the end portions of one polepiece confront the end portions of the other pole piece, a shim embeddedin said support and extending radially therefrom and having a portion inabutting engagement with adjacent end portions of the pole pieces toprovide a gap of definite length therebeteween, and a boss integral withsaid support and projecting radially therefrom and in engagement withother adjacent end portions of the pole pieces to separate said otherend portions and to maintain the first-mentioned end portions inengagement with the shim.

8. In a pole-piece unit for magnetic heads, a pair of magneticsubstantially C-shaped pole pieces having end portions arranged inadjacency, a substantially cylindrical non-magnetic support inconcentric engegement with the inner curvature of said pole pieces, anon-magnetic shim of predetermined thickness disposed to extend betweenadjacent end portions of said pole pieces and in abutting engagementtherewith to define a gap of definite length therebetween, and aprojection of predetermined width extending radially of said support andin engagement with other adjacent end portions of the pole pieces toeffect abutting engagement between the first-men- 7 tioned end portionsof the pole pieces and the shim.

9. A support for generally C-shaped pole pieces of a magnetic head, saidsupport consisting of a body having non-magnetic rounded surfaceportions adapted to receive such pole pieces and to align end portionsthereof, and a pair of gap defining non-magnetic extensions rediatingfrom said body for abutting engagement with said end portions of thepole pieces when received on the mentioned surface portions of saidbody.

10. A magnetic head comprising a yoke structure including a magneticcore with a gap therein, said gap having a generally circular shapedportion and a generally rectangular shaped portion, the latter providingthe core With end sections disposed in generally parallel confrontingrelationship, a generally cylindrical pole-piece unit mounted in saidcircular shaped portion of the gap and including a pair of substantiallyC- shaped pole pieces and a generally cylindrical member supporting saidpole pieces with their end portions in adjacency, a shim disposed onsaid support and extending radially therefrom and in engagement withadjacent end portions of the pole pieces to establish a definite spacingtherebetween, a boss disposed on said support and projecting radiallytherefrom and in engagement with other adjacent end portions of the polepieces to separate said other end portions and to efiect seatingengagement between the first-mentioned end portions and the shim, and anelement provided on said boss and extending in the rectangular portionof the gap between the mentioned end sections of the core to prevent thepole-piece unit from rotating be yond desired limits in said circulargap portion.

11. For a magnetic head including a magnetic core provided with a gaphaving a generally circular portion and a generally rectangular portion,the latter providing the core with end sections disposed in generallyparallel confronting relationship, a generally cylindrical pole-pieceunit mountable in said circular portion of the gap and including a pairof substantially C- shaped pole pieces and a generally cylindricalmember supporting said pole pieces with their end portions in adjacency,a shim disposed on said member and extending radially therefrom and inengagement with adjacent end portions of the pole pieces to establish adefinite spacing therebetween, a boss disposed on said support andprojecting radially therefrom and in engagement with other adjacent endportions of the pole pieces to separate said other end portions and toeffect seating engagement between the firstmentioned end portions andthe shim, and an element provided on said boss and adapted for insertionin the rectangular portion of the gap between the end sections of thecore to guide the mounting of the pole-piece unit in the circularportion of the gap and to prevent the pole-piece unit, when so mounted,from rotating beyond desired limits in relation to the circular gapportion.

WILLIAM P. WEST.

REFERENCES CITED The following references are of record in the file ofthis patent:

' UNITED STATES PATENTS

